The hot, dry weather that has predominated in some areas of Illinois has enabled twospotted spider mites to gain a foothold in some soybean fields. We shared our first report of the occurrence of spider mites in southern Illinois (Hamilton County) this year in issue no. 9 (May 25, 2001) of the Bulletin. Now we've learned that some fields are being treated for control of spider mites. Dennis Epplin, crop systems educator at the Mt. Vernon Extension Center, reported on July 1 that fields in Gallatin and White counties have been sprayed for control of spider mites. Robert Bellm, crop systems educator at the Edwardsville Extension Center, was in a field in White County that had been treated for spider mites on June 28. And spider mites are showing up elsewhere, too. Russ Higgins, IPM educator at the Matteson Extension Center, found twospotted spider mites in a no-till soybean field on sandy soil in Grundy County on July 1. The plants were suffering from a lack of moisture.|
Unless we get some rainfall in dry areas of the state, twospotted spider mites probably will become established and their population densities will increase. Therefore, we strongly encourage you to start looking for these pests that are notoriously associated with droughty conditions. Spider mites have tremendous reproductive capabilities and can go from "0 to 60" rather quickly. A few mites here and there can explode into a noticeable infestation in a very short time.
Start watching for spider mites in soybeans along field margins and in especially dry areas of fields. Plants injured by spider mites will be stunted, and leaves will appear stippled and then entirely yellow. Another telltale sign of the presence of spider mites is webbing on the undersides of the leaves. Although spider mites are extremely small (0.3 to 0.4 mm), you can see them fairly easily with a good hand lens. Also, if you tap infested leaves over a sheet of paper, you will be able to see mites crawling on the paper.
The treatment guideline for twospot-ted spider mites in soybeans is somewhat cryptic and not based on extensive research. However, a miticide may be warranted if 20 to 25% of the plants are discolored and mites are present. Products suggested for control of spider mites in soybeans are dime-thoate (see product label for rate) and Lorsban 4E at 1/2 to 1 pt per acre. The use of Lorsban 4E is restricted to certified applicators.--Kevin Steffey